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Dental zirconia restorations present long-term clinical survival and stay in service within the oral environment for many years. However, low temperature degradation could affect their mechanical properties and survival. Objectives: To investigate the effect of two aging methods on the flexural strength and crystal structure of yttrium-stabilized zirconia (YTZP). MATERIALS AND METHODS: Thirty bar specimens were prepared from a Yttria stabilized zirconia polycrystals and were divided into 3 groups (control, aged for 720,000 mechanical loads of 50N and 3600 thermal cycles, aged for 1 hour using autoclave). The aging procedures represent 3 years of clinical use. The specimens were loaded until fracture and the crystalline phase polymorphs of the material (tetragonal, t, and monoclinic, m, zirconia) were investigated by x-ray diffraction (XRD). Further investigations were done using scanning electron microscope (SEM). Data was statistically analysed using ANOVA test. RESULTS: Group B and C showed no statistical significance in their flexural strength with means of their break force (793.23±164.03) and (780.97 ± 257.25) respectively but statistically significant and higher than group A with mean (549.7 ± 54.14). The XRD showed nearly no change in the crystal structure between group A and B but an increase in the percent of monoclinic phase in group C. The SEM demonstrated a relatively homogenous size with particle size ranged between 400 to 570 µm for group A, while Group B and C showed an increase in particle size between 768 to 1150 µm respectively. CONCLUSIONS: Both aging methods caused changes in the flexural strength and structure of the zirconia specimens with no significant difference between them. KEYWORDS: Fixed prosthodontics, Zirconia, Aging, Low temperature degradation.

THE EFFECT OF TWO AGING METHODS ON THE FLEXURAL STRENGTH AND CRYSTAL STRUCTURE OF YTTRIA STABILISED ZIRCONIA POLYCRYSTALS (IN VITRO STUDY)